Controlling the motion of nano- and micro-scale particles and objects has been a long-sought goal in science and engineering. These functional particles, also referred to as micro-robots, micro-swimmers, or nano-motors, can have a wide range of applications, including biology, medicine, microfluidics and colloidal science.
Conventional methods of controlling the motion of nano- and micro-scale particles usually rely on chemical, electric, magnetic, acoustic, and temperature effects to power the transport of the particles. However, these methods usually suffer the drawbacks of failing to provide controllable and high-speed movement, poor biocompatibility, and little to no ability to operate in biologically relevant environments.
Light can also be used to transport and guide particles of sizes that are substantially similar to or less than the wavelength of the light. Examples of optical guiding include optical tweezers and optical tractor beams. However, these approaches typically include beam shaping to realize complex electromagnetic field profiles and are thus sensitive to scattering.
Light-induced thermal effects can be employed to address the sensitivity to scattering. For example, in a metal-dielectric particle (e.g., a Janus particle), the heat generated by the absorption of light in the metal side can induce a local temperature difference, resulting in propulsion (i.e., thermophoresis) along the axis of the temperature gradient. Because the thermophoretic drift is based on absorption of light, it can be robust to scattering in the surrounding environment. However, thermophoretic drift typically points in the same direction. Therefore, it can be challenging to guide or steer the particle along other directions, thereby rendering it difficult for the particle to reach an arbitrary target location. In existing thermophoretic guiding schemes, the particle guided by light-actuated thermophoresis is usually monitored in real time and actuated by light only when its orientation satisfies a certain condition (e.g., when the particle is facing toward the target location). This results in slow guiding speeds and the need for complex optical instrumentation.